Recently, a water absorbent resin is widely used as a main component of sanitary materials (absorbent articles) such as paper diapers, sanitary napkins, incontinence pads and the like, in order to absorb body fluids (e.g. urine and blood).
Well-known examples of the water absorbent resin are (i) cross-linked partially neutralized polyacrylic acid; (ii) a hydrolyzed starch-acrylonitrile graft polymer; (iii) a neutralized starch-acrylic graft polymer; (iv) a saponified vinyl acetate-acrylic ester copolymer; (v) cross-linked carboxymethylcellulose; (vi) hydrolyzed acrylonitrile copolymer or hydrolyzed acrylamide copolymer, or cross-linked acrylonitrile copolymer or cross-linked acrylamide copolymer; (vii) a cross-linked cationic monomer, (viii) a cross-linked isobutylene-maleic acid copolymer; (ix) a cross-linked body of 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid; (x) and the like.
The water absorbent resin has a high absorbency with respect to an aqueous liquid. However, due to its high absorbency, the water absorbent resin absorbs also moisture in air (moisture absorption). Thus, particles of the water absorbent resin are coagulated with each other, which results in a blocking phenomenon in which the water absorbent resin loses fluidity as powder. The moisture absorption blocking phenomenon raises the following problems: at the time of production of the water absorbent resin and at the time of production of a sanitary material such as a disposable paper diaper using the water absorbent resin, the blocking phenomenon occurs in a storage hopper or a transportation line, or the water absorbent resin adheres to a production apparatus, so that the blocking or adhering water absorbent resin becomes much less treatable. As a result, it is impossible to stably manufacture the product. Then, in order to prevent the blocking phenomenon occurring at the time of moisture absorption, a method in which an inorganic compound is added to the water absorbent resin is conventionally adopted.
Examples of the method are: a method in which an inorganic powder is mixed with such a water absorbent resin that approximately 60 wt % or less polymer particles pass through a sieve of 300 μm in mesh (Published Japanese Translations of International Publication of Patent Application No. 523526/2002 (Tokuhyo 2002-523526)); a water-insoluble water absorbent resin composition obtained by adding water to a mixture of multivalent metal salt and a water absorbent resin (Japanese Examined Patent Publication of Patent Application No. 4667/1992 (Tokukohei 4-4667)); a production method in which water containing a multivalent metal salt is sprayed to a water absorbent resin (Japanese Examined Patent Publication of Patent Application No. 40780/1993 (Tokukohei 5-40780)); a water absorbent resin modified by adding water containing a multivalent metal salt to a surface of a water absorbent resin whose particle size ranges from 5 to 500 μm and by heating the water absorbent resin (Japanese Unexamined Patent Publication No. 46241/1986 (Tokukaisho 61-46241); a water absorbent resin modified by adding water, in which inorganic salt has been dissolved, to a water absorbent resin whose surface has been cross-linked (Japanese Unexamined Patent Publication No. 124879/1997 (Tokukaihei 9-124879)); a water absorbent agent, obtained by adding a multivalent metal compound to a water absorbent resin, in which multivalent metal locally exists in a vicinity of a surface of the water absorbent agency (Japanese Unexamined Patent Publication No. 96151/2001 (Tokukai 2001-96151)); and the like.
Further, examples of a method in which an inorganic compound is added to the water absorbent resin in order to improve a water-absorbing ability of the water absorbent resin powder include: a method in which a multivalent metal salt such as aluminum sulfate and a water absorbent resin are dry-blended and thus obtained mixture is brought into contact with a binding agent (water or the like) so as to produce such a water absorbent resin that a gel having absorbed water has elasticity and a gel blocking phenomenon hardly occurs (Published Japanese Translations of International Publication of Patent Application No. 523289/2001 (Tokuhyo 2001-523289)); a composition obtained by adding an aqueous solution containing multivalent metal salt to a water absorbent resin that has been thermally processed at more than 170° C. for more than 10 minutes (Published Japanese Translations of International Publication of Patent Application No. 523287/2001 (Tokuhyo 2001-523287)); a water-insoluble swelling hydrogel obtained by coating a water-insoluble swelling hydrogel, having specific absorbency under pressure and having gel strength, with a three-dimensional or electrostatic spacer (U.S. Patent No. 2002/0128618); a water absorbent resin secondarily cross-linked by an organic surface cross-linking agent aqueous solution and a cation aqueous solution (Published Japanese Translations of International Publication of Patent Application No. 538275/2002 (Tokuhyo 2002-538275)); and the like.
However, according to these known methods, it is impossible to sufficiently improve a moisture absorption blocking property, and an absorbency and a diffusing absorbency under pressure may significantly drop. Further, it is impossible to sufficiently improve an anti-moisture-absorption-blocking property while suppressing drop in the absorbent properties (for example, an absorbency, a diffusing absorbency under pressure, and the like) of the water absorbent resin.
Recently, the sanitary material such as the sanitary napkin and the like using the water absorbent resin has higher performance and a thinner size, and an amount of the water absorbent resin used for each sanitary material tends to increase, and also a weight % of the water absorbent resin tends to increase with respect to the whole absorber constituted of the water absorbent resin and a hydrophilic fiber. That is, by using (i) a smaller amount of a hydrophilic fiber whose bulk density is low and (ii) a larger amount of a water absorbent resin having a superior water absorbent property and high bulk density, a ratio of the water absorbent resin contained in the absorber is increased, thereby making the sanitary material thinner without decreasing an amount of water absorption.
The sanitary material which includes a smaller amount of the hydrophilic fiber and a larger amount of the water absorbent resin is preferable merely in terms of liquid storage, but raises problems in terms of distribution and diffusion of liquid in actual use in diapers.
That is, when a large amount of the water absorbent resin is used, the water absorbent resin becomes soft and gelatinous upon absorbing water. This causes a gel blocking phenomenon. As a result, a liquid diffusing property of the diaper significantly drops. In order to avoid such phenomenon and to keep the absorbent property of the absorber high, a ratio of the hydrophilic fiber and the water absorbent resin is limited, so that there is a limit in making the sanitary material thinner.
As means for improving the liquid diffusing property of the diaper while preventing the gel blocking in the diaper, there are proposed: a method in which two types of water absorbent resins different from each other in terms of the water absorbent property are used (Japanese Unexamined Patent Publication No. 252307/2001 (Tokukai 2001-252307)); a method in which a composition containing a cationic ion exchange hydrogel formation polymer and an anionic ion exchange hydrogel formation polymer is used (WO 98/37149 pamphlet); a method in which a water absorbent resin having high cross-linking density in its surface is used (Japanese Unexamined Patent Publication No. 057010/1994 (Tokukaihei 6-057010)); and the like. However, in these methods, there is a room for improving the absorbent property as an absorber having a high water absorbent resin density.
Further, it is well known that a water absorbent resin is treated with a metal compound so as to improve a liquid diffusing property of water absorbent resin powder. Examples of the method include: a method in which multivalent metal salt such as aluminum sulfate is dry-blended with a water absorbent resin, and is brought into contact with a binding agent (water or the like), thereby producing a water absorbent resin, having elasticity, which hardly causes the gel blocking (WO 98/48857 pamphlet); a method in which a water absorbent resin is secondarily cross-linked with a water-soluble organic surface cross-linking agent and water-soluble cation (Tokuhyo 2002-538275 and Tokuhyo 2002-539281); a high water absorbent resin composition which includes a high water absorbent resin and fine powder made up of hydrated oxides at least partially containing two types of metals (M1 and M2) so as to have a -M1-O-M2- bond (Japanese Unexamined Patent Publication No. 147724/1998 (Tokukaihei 10-147724)); and the like.
In accordance with the various known methods, efforts have been made so as to improve (i) the gel blocking property or (ii) liquid permeability/liquid diffusing property. However, a water absorbent resin which improves these two properties (the gel blocking property and the liquid permeability/liquid diffusing property) at the same time has not been invented.
Further, in order to produce a water absorbent resin composition which is superior in the gel blocking property and the liquid permeability/liquid diffusing property, it is necessary that surfaces of particles of the water absorbent resin are secondarily cross-linked evenly. When the surfaces of the particles are not secondarily cross-linked evenly, portions which are not secondarily cross-linked to each other are clumped, so that the blocking phenomenon occurs. As a result, a production apparatus is blocked up, so that it is impossible to stably produce the water absorbent resin.